Australia could take until well into next year to hit an 80% booster vaccination rate, with the rollout currently ranking almost last out of 70 countries, Guardian analysis shows.
Boosters are now available to anyone who has had a second dose of a Covid vaccine more than five months ago.
However, Australia's slow initial vaccine rollout meant that more than half of all second doses were administered in the final four months of 2021. So with a five-month dosing interval, the people who received their second doses late in 2021 won't be eligible for a booster until February.
Experts warn that a shorter interval between second and booster doses could mean the immune response is not as high or long lasting.
"We do know from evidence from the first two doses that a longer interval between the first dose and second dose gives you higher antibodies, higher protection," says Cyra Patel, a PhD candidate at the Australian National University.
"And that is consistent with our knowledge of immunology as well. We have seen that with other vaccines a longer interval gives you a better boosting effect. It's really a balance between getting you protection from another dose earlier and giving you better protection if you have a longer interval."
Analysis of vaccination data shows that roughly the same number of people have now had booster shots as were fully vaccinated about 170 days ago. This means the booster rate is still on track with a six-month interval.
The move to a five-month gap has increased the daily booster rate from about 20,000 a day to more than 100,000. But the Christmas break could see this rate slow again as distribution points close or scale down.
Australia's booster shot program kicked off at the beginning of November, initially targeting those in residential aged care and disability facilities. Booster shots were available to anyone over the age of 18 who had received their second dose at least six months prior.
Patel says the initial six-month interval came from the clinical trials.
"That's the time period that the clinical trials use. So we have got clear evidence and that has been reviewed by the TGA and other regulators that definitely at six months you are definitely going to see a boosting effect."
The Australian Technical Advisory Group on Immunisations (Atagi) recently reduced the recommended period between doses from six to five months. Other countries have cut the dosing interval even further in response to the Omicron variant, either because there is more virus circulating in their communities or because they are heading into winter.
Both Ireland and Sri Lanka are administering boosters to those who had a second shot at least three months ago.
"Some of those decisions about shortening it to three months are based on expert opinion about the amount of disease that is circulating, about creating access to vaccines and taking away some of the barriers to getting the booster dose," says Patel.
"But there really is not much evidence that a booster dose at three months is better than six months."
Israel and Iceland are building on their initially successful vaccine rollouts, and have already administered boosters to more than 60% of the fully vaccinated population. Australia has administered Covid boosters to 5% of the population.
Of the almost 70 countries for which there is data on booster shots, Australia ranks near the bottom.
Vaccination data shows fewer than 170,000 people had been fully vaccinated at least six months before the booster program began. Since then the number of people receiving boosters has tracked roughly in line with the initial vaccination rollout.
The government says reducing the gap to five months means that almost 4 million people will be eligible to receive a booster dose by the end of the year. But Australia will come nowhere close to this mark at the current rate of around 100,000 boosters administered per day.
